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pbs681
Joined: 19 Aug 2004
Posts: 154
Helped: 11
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 20 Sep 2008 6:28 How to set Drain voltage (Vd) |
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Hi All,
We know that Id=K'w/l(Vgs-Vth)^2 (saturation- FIRST ORDER). From the equation we can get voltage at Vg and Vs depending on the info that we have (i.e I is fixed or w/l is fixed). From the above equation, I don't see Vd term in the equation. So, my question is that how can we set Vd voltage in our circuitry.
Thanks in advance.
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starcome
Joined: 09 Nov 2005
Posts: 8
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| 20 Sep 2008 10:49 Re: How to set Drain voltage (Vd) |
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| pbs681 wrote: |
Hi All,
We know that Id=K'w/l(Vgs-Vth)^2 (saturation- FIRST ORDER). From the equation we can get voltage at Vg and Vs depending on the info that we have (i.e I is fixed or w/l is fixed). From the above equation, I don't see Vd term in the equation. So, my question is that how can we set Vd voltage in our circuitry.
Thanks in advance. |
Id=K'w/l(Vgs-Vth)^2 is based on the assumption that Vgs-vth<=Vds.So Vd>Vg-vth.
Futhermore, his equation only refects first order, vds will also effect I by channel modulation effect and Vth, ro, gain by NIBL, etc.
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pbs681
Joined: 19 Aug 2004
Posts: 154
Helped: 11
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| 20 Sep 2008 14:01 Re: How to set Drain voltage (Vd) |
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| starcome wrote: |
| pbs681 wrote: |
Hi All,
We know that Id=K'w/l(Vgs-Vth)^2 (saturation- FIRST ORDER). From the equation we can get voltage at Vg and Vs depending on the info that we have (i.e I is fixed or w/l is fixed). From the above equation, I don't see Vd term in the equation. So, my question is that how can we set Vd voltage in our circuitry.
Thanks in advance. |
Id=K'w/l(Vgs-Vth)^2 is based on the assumption that Vgs-vth<=Vds.So Vd>Vg-vth.
Futhermore, his equation only refects first order, vds will also effect I by channel modulation effect and Vth, ro, gain by NIBL, etc. |
yes, this is first order eq. Lets forget about second order effect for learning purpose. So, yes, we have to makesure Vds>=Vgs-Vth for sat region operation. But if we use the equation above we will get vds that on the boundary of the sat and linear region which is quite risky because the transistor can leave the sat region anytime due to offset or mismatch. So, how big is normally we need to set Vds from Vdsat for safer operation?
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sutapanaki
Joined: 02 Nov 2001
Posts: 488
Helped: 19
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| 20 Sep 2008 19:48 Re: How to set Drain voltage (Vd) |
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| Apart from the equation, which gives only approximate relation between the current and the overdrive voltage, the drain voltage is very much dependent on the type of circuit you have at hand. A transistor by itself and in saturation is a current source with a relatively high output impedance. If attached to the drain of that transistor you have a lower impedance circuit, then it will define the drain voltage. If, instead, there is another high impedance circuit connected to the drain of the transistor, then the drain voltage is pretty much dependent on the amount of current mismatch between the current sunk by your transistor and sourced by the circuit attached to its drain. This delta I will effectively go into the equivalent impedance at the drain and will define the voltage. Most often, for high impedance levels the drain voltage will rail either to Vdd or Gnd. That is the reason why people use in these kind of cases CMFB circuits or any other kinds of negative feed-back.
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lsimeon
Joined: 01 Aug 2008
Posts: 43
Helped: 2
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| 23 Sep 2008 12:37 How to set Drain voltage (Vd) |
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how can we set Vd voltage in our circuitry.
--> actually, there are parameters that can affect the value of Vd (Rd and Rs) supposed that we ignore the internal parameters that affects the value of Id..
1.) If there is Rd w/o Rs, Vd = Vdd-IdRd; since the device is in saturation, Vd is approx. equal to zero
2.) there is Rs w/o Rd, Vd = Vdd
3.) there are Rd and Rs, Vd = Vdd-IdRd or approx. zero
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pbs681
Joined: 19 Aug 2004
Posts: 154
Helped: 11
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| 23 Sep 2008 12:46 Re: How to set Drain voltage (Vd) |
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| sutapanaki wrote: |
| Apart from the equation, which gives only approximate relation between the current and the overdrive voltage, the drain voltage is very much dependent on the type of circuit you have at hand. A transistor by itself and in saturation is a current source with a relatively high output impedance. If attached to the drain of that transistor you have a lower impedance circuit, then it will define the drain voltage. If, instead, there is another high impedance circuit connected to the drain of the transistor, then the drain voltage is pretty much dependent on the amount of current mismatch between the current sunk by your transistor and sourced by the circuit attached to its drain. This delta I will effectively go into the equivalent impedance at the drain and will define the voltage. Most often, for high impedance levels the drain voltage will rail either to Vdd or Gnd. That is the reason why people use in these kind of cases CMFB circuits or any other kinds of negative feed-back. |
how if the circuitry didn't attached to any other circuitry. ie cascode configuration
Added after 1 minutes:
| lsimeon wrote: |
how can we set Vd voltage in our circuitry.
--> actually, there are parameters that can affect the value of Vd (Rd and Rs) supposed that we ignore the internal parameters that affects the value of Id..
1.) If there is Rd w/o Rs, Vd = Vdd-IdRd; since the device is in saturation, Vd is approx. equal to zero
2.) there is Rs w/o Rd, Vd = Vdd
3.) there are Rd and Rs, Vd = Vdd-IdRd or approx. zero |
how if we only have transistor in our circuitry... no resistor at drain or source....
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sutapanaki
Joined: 02 Nov 2001
Posts: 488
Helped: 19
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| 23 Sep 2008 18:36 Re: How to set Drain voltage (Vd) |
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| In my previous post by any circuitry I really meant ANY. A cascode transistor is also in this class. Moreover, the cascode transistor asserts a low impedance connection at the drain of the bottom device and as such it will define the drain voltage as I explained before. Now, here I really assume you mean there is a cascode transistor connected to the drain of your original transistor and you want to know this drain voltage. If so, you just have to choose the correct voltage at the gate of your cascode device. Looking from that gate, the drain of the bottom device (at least in DC) is in a similar situation as in a source follower. Based on the current and your threashold voltage you can find the Vgs of the cascode device and subtracting it from the gate voltage will give you the drain voltage. Well, more or less:-)
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pbs681
Joined: 19 Aug 2004
Posts: 154
Helped: 11
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| 24 Sep 2008 1:46 Re: How to set Drain voltage (Vd) |
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| sutapanaki wrote: |
| In my previous post by any circuitry I really meant ANY. A cascode transistor is also in this class. Moreover, the cascode transistor asserts a low impedance connection at the drain of the bottom device and as such it will define the drain voltage as I explained before. Now, here I really assume you mean there is a cascode transistor connected to the drain of your original transistor and you want to know this drain voltage. If so, you just have to choose the correct voltage at the gate of your cascode device. Looking from that gate, the drain of the bottom device (at least in DC) is in a similar situation as in a source follower. Based on the current and your threashold voltage you can find the Vgs of the cascode device and subtracting it from the gate voltage will give you the drain voltage. Well, more or less:-) |
Thanks sutapanaki.... Now I understand it. Thanks again !
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pbs681
Joined: 19 Aug 2004
Posts: 154
Helped: 11
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| 29 Sep 2008 15:29 Re: How to set Drain voltage (Vd) |
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| sutapanaki wrote: |
| Apart from the equation, which gives only approximate relation between the current and the overdrive voltage, the drain voltage is very much dependent on the type of circuit you have at hand. A transistor by itself and in saturation is a current source with a relatively high output impedance. If attached to the drain of that transistor you have a lower impedance circuit, then it will define the drain voltage. If, instead, there is another high impedance circuit connected to the drain of the transistor, then the drain voltage is pretty much dependent on the amount of current mismatch between the current sunk by your transistor and sourced by the circuit attached to its drain. This delta I will effectively go into the equivalent impedance at the drain and will define the voltage. Most often, for high impedance levels the drain voltage will rail either to Vdd or Gnd. That is the reason why people use in these kind of cases CMFB circuits or any other kinds of negative feed-back. |
can we say that this is a loading effect??
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sutapanaki
Joined: 02 Nov 2001
Posts: 488
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| 29 Sep 2008 17:52 Re: How to set Drain voltage (Vd) |
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| In a way yes, although it is good to know what you understand by a loading effect. Usually, and in my view, when one says something is a loading effect I understand that the circuit was intended to work in a certain way but because of the effect of the load (which ideally is not accounted for), the operation is not quite as initially intended. Which certainly is not the case here.
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